As the permanent-magnet rotor of an electric machine rotates, it induces a back emf in a winding of the electric machine. As the rotor accelerates, the magnitude of the back emf increases. It therefore becomes increasingly difficult to drive power into the electric machine. In order to overcome this difficulty, the winding may be excited in advance of the back end.
Since the back emf increases with the speed of the rotor, the advance angle at which excitation leads the back emf ideally increases with the rotor speed. However, this then requires on-the-fly calculations in order to calculate the required advance angle for the speed of the rotor, and then to calculate the excitation times on the basis of the advance angle and the rotor speed.